MdVQ37 Negatively Regulates Apple Resistance to Valsa Canker via SA-Dependent and SA-Independent Pathways

Apple Valsa canker is one of the major fungal diseases adversely affecting the apple industry. Valine-glutamine motif-containing proteins (VQs) are a kind of plant transcription regulation cofactor, which are involved in regulating the growth and development of plants and their defence responses. Yet little is known about the role of VQs in the biotic stress response of woody plants, especially in apple trees. Recently, we identified MdVQ37 as a negative regulator of apple resistance to Glomerella leaf spot by inhibiting MdWRKY100-mediated salicylic acid (SA) accumulation via protein interaction. Here we found that MdVQ37 expression was induced significantly by Valsa mali infection. Overexpressing MdVQ37 in apple increased its susceptibility to V. mali, accompanied by a reduction in H₂O₂ accumulation along with the activity of superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), chitinase and β-1,3-glucanase. Meanwhile, MdVQ37 overexpression increased the expression of two SA catabolic genes, MdS5H1 and MdS5H2, resulting in a greater accumulation of the SA metabolite 2,5-dihydroxybenzoic acid (2,5-DHBA), but a decreased SA content and less signalling in transgenic plants. MdPER3, a peroxidase-encoding gene, was identified as a direct target of MdWRKY100. Analysis of transcriptional regulation activity showed that MdVQ37 inhibited the binding and transcriptional ability of MdWRKY100 on the MdPER3 promoter, thereby reducing antioxidant enzyme activity and weakening the Valsa canker resistance. Overall, our results show that MdVQ37 negatively regulates apple defence against V. mali through both SA-dependent and SA-independent pathways. Our findings provide novel insight into the mechanisms by which the VQ-WRKY complex modulates plant defence responses against pathogens.